The environmental authorities are placing ever greater demands on the pulp industry to decrease the use of chlorine gas for bleaching. Permitted discharges of organic chlorine compounds (AOX) with the waste water from bleaching plants have been gradually decreased and are now at such a low level that the pulp factories have in many cases stopped using chlorine gas. Instead, only chlorine dioxide is used as a bleaching agent. Chlorine dioxide forms smaller quantities of AOX than chlorine gas while achieving the same bleaching effect.
However, even the use of chlorine dioxide has been questioned. On the one hand, the environmental authorities in certain countries demand that the discharges of organic chlorine compounds be reduced to such a low level that these demands can scarcely be met even if only chlorine dioxide is used for bleaching. On the other hand, in addition, customers in many countries have begun to demand paper products which are bleached entirely without using either chlorine gas or chlorine dioxide.
The pulp industry is therefore searching for methods which permit bleaching of pulp without using chlorine chemicals. Methods which have been successfully tested involve removing metals in an acid stage (A stage), or possibly by addition of chelating agents (Q stage), e.g. ethylenediamineteteacetic acid (EDTA), to an oxygen-delignified pulp. The pulp is washed and is further bleached using, for example, hydrogen peroxide (P) and/or ozone (Z) in different sequences. One example is the method which is described in European Patent 90850200 (Swedish Patent SE-A-8902058) of EKA NOBEL, the so-called Lignox method. Another known bleaching process includes the bleaching sequences AZ (EOP: alcaline extraction, oxygen, peroxide) where A is an acid stage without use of a chelating agent and Z is as described above.
It is a significant feature of these different methods that certain metal ions have a negative effect on the bleaching process in the form of impaired pulp quality and/or greater consumption of chemicals. In these methods, it has been the practice to wash out the metals by means of an open A/Q stage. A problem which arises in conjunction with these methods is that, as a result, a liquid flow is obtained from the washing stage after release of the metals which contains, on the one hand, a certain amount of released substances which may be toxic or cause pollution and, on the other hand, include dissolved metal ions, which situation is difficult to manage from the point of view of waste and recovery. According to conventional technology, this filtrate is treated by means of external purification, which can be complicated, time consuming and costly, after which the filtrate is released into a receiver.